The ?4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for Alzheimer's disease (AD), which is pathologically defined by the presence of amyloid- (A)-containing plaques and hyperphosphorylated tau-containing neurofibrillary tangles. APOE4 is also a major genetic risk factor for cerebral amyloid angiopathy (CAA), a common pathological feature of AD with amyloid deposits along the cerebrovasculature. Our long-term goal is to understand how APOE4 differs from APOE3 and APOE2 in regulating A metabolism and the formation of amyloid plaques and CAA, thereby increasing risk for AD and CAA. As apoE is expressed abundantly both in brain parenchyma by astrocytes and in the cerebrovasculature by vascular mural cells, which include smooth muscle cells and pericytes, it is critical to examine how apoE isoforms expressed in different domains of the brain regulate apoE-related biology and pathobiology. As such, we have generated inducible and cell-type specific mouse models that express human apoE3 or apoE4 with the major goal being to test the specific roles of apoE isoforms produced by astrocytes or vascular mural cells in BBB permeability, brain A clearance and the formation of amyloid plaques and CAA. We hypothesize that human apoE4 expressed both in astrocytes and vascular mural cells contributes to compromised BBB integrity, impaired brain A clearance and the formation of amyloid plaques and CAA. We propose three complementary aims to test our hypothesis. In Aim 1, we plan to compare how apoE isoforms produced by astrocytes or vascular mural cells differ in their biochemical properties and functions in regulating receptor binding, lipid transport, BBB integrity and A cellular uptake. In Aim 2, we plan to examine how expression of apoE3 or apoE4 in astrocytes or vascular mural cells affects BBB integrity, brain A clearance, amyloid plaque deposition, and the formation of CAA in vivo using our recently developed mouse models that allow for inducible and cell-type specific expression of human apoE3 or apoE4. Finally in Aim 3, we will analyze how apoE isoforms and their expression levels influence the severity and topographical distribution of CAA in humans using a large collection of autopsy brains available through the Mayo Clinic ADRC Neuropathology Core. Together, our studies using cellular and animal models, as well as human autopsy brain tissue, will allow us to elucidate how apoE isoforms expressed in brain parenchyma and in cerebrovasculature regulate brain A clearance and the formation of amyloid plaques and CAA. These studies also have the potential to generate novel insights into how we can design therapeutic strategies for AD and CAA by targeting apoE.